In order to detect conditions under which vehicles are moving along a road, the technique of an infrastructure-mounted radar or the like has traditionally been used. For example, a radar, such as a milliwave radar, is mounted at a position which overlooks a road for detecting moving vehicles.
For example, the use of such a technique makes it possible to detect traffic conditions on a road, such as vehicles moving along the road, traffic congestion caused due to temporary stopping of a vehicle or long stopping of a vehicle due to an accident or the like, and measured traffic volume, in order to manage road conditions. In addition, it is possible to inform succeeding vehicles and the like of the road conditions by the use of a display unit or the like mounted beside the road, in order to prevent an accident.
Furthermore, a technique for detecting a variation of radar mounting by the use of two radars is known as a technique for detecting a positional variation of vehicle-mounted radar mounting. In this case, one radar performs detection in an upper area and the other radar performs detection in a lower area. These areas overlap and the overlap area is parallel to a road surface (see, for example, Japanese Laid-open Patent Publication No. 11-14747). With this technique a moving object such as another vehicle is to be detected in the overlap area if the radars are mounted in normal positions. On the other hand, if a radar changes its angle upward or downward, a moving object is also detected outside the overlap area, which means a variation of radar mounting.
An infrastructure-mounted radar, for example, may be used for a decade or even longer. Therefore, it is desirable that radar maintenance be unneeded. From this point of view, a scannerless radar in which an angle between a transmission antenna plane and a receiving antenna plane is fixed and does not need changing at vehicle detection time is preferable to a mechanical scan type radar in which an angle between a transmission antenna plane and a receiving antenna plane is physically changed at vehicle detection time. Furthermore, in scannerless radars, radars using an antenna beam of narrow width (narrow beam radars) are preferable because they have higher receiving power than wide beam radars and are capable of sharply detecting the position of an object.
Such a narrow beam radar, however, may fail to cover a desired detection area due to a variation of angle of the radar. Therefore, the angle of radar mounting needs to be checked if it is always correct.
The technique disclosed in the above Japanese Laid-open Patent Publication No. 11-14747 uses two radars for detecting a positional variation although one radar is sufficient for the original purpose, which is not economical.
In addition, it is desirable from the viewpoint of easy maintenance that the occurrence of an abnormality of a radar, such as a variation of an mounting angle, is detected only based on data outputted therefrom without actually inspecting the radar.